In 1971 a research group at Stanford University made the first demonstration of a terahertz source, based on the optical parametric technique. Since then, other groups have contributed to the improvement of the optical, parametric terahertz-beam generation. While this research has clearly improved the properties of nonlinear optical materials, used in terahertz parametric sources, only marginal progress has been made on the technique itself of parametric terahertz-beam generation.
The parametric generation of sub-harmonic frequency signals can be achieved, if the following two conditions are satisfied:ω3=ω1−ω2  (1); andk3=k1−k2.  (2)
Here, ω1, ω2 and ω3 are the frequencies of the pump beam, the idler beam and the signal beam, respectively; and k1, k2 and k3 are the wave vectors of the pump beam, the idler beam and the signal beam, respectively. The idler beam is produced by the Raman process, which occurs inside a nonlinear crystal, when the pump beam interacts with the nonlinear crystal. The first equation shows the frequency matching condition (see FIG. 1), which corresponds to energy conservation, and the second equation shows the phase matching condition, which corresponds to momentum conservation.
When a high-frequency electromagnetic beam (e.g., an infrared or visible light beam) traverses a medium and/or the nonlinear optical crystal, the high frequency beam experiences the phenomenon of dispersion. As a result, the refractive index n, which is frequency dependent within the medium and/or crystal, and the wave vector k=n(ω) ω/c (3)
would not necessarily change linearly with the frequency ω. For this reason, in general, it is not possible to satisfy both the frequency matching and the phase matching conditions within a crystal. This difficulty can be overcome by exploiting the birefringence in an anisotropic crystal, so that both the frequency matching condition and the phase matching condition can be achieved. Birefringence implies that there are two different refractive indices, n(e) and n(o), where n(e) denotes the refractive index along the extraordinary optical axis, and n(o) denotes the refractive index along the ordinary axis. Because the wave vectors are related to the refractive indices by k=n(ω) ω/c, there are two wave vectors, k(e) and k(o), along the extraordinary and ordinary axes. Also, because the values of k(e) and k(o) are usually different, as well as dependent on the angle of incidence of the pump beam 102 (see FIG. 1), the phase matching condition illustrated in FIG. 1 can be achieved, either by type I phase matching or by type II phase matching.Type I phase matching: k1(e)=k2(o)+k3(o); and  (4)Type II phase matching: k1(e)=k2(e)+k3(o) or k1(e)=k2(o)+k3(e).  (5)
FIG. 1 shows a diagram of Type I phase matching, in which the birefringence of a nonlinear crystal is used to compensate for the dispersion in the material.
This technique is widely used in optical parametric beam generation. For terahertz beam generation the same technique can be employed, and two different types of terahertz sources have been developed. These two different types of parametric terahertz sources that have been developed are (1) the terahertz parametric generator (TPG) and (2) the terahertz parametric oscillator (TPO). The structure of the TPG is similar to that of the TPO, except that the TPG has no resonating cavity for idler beam 104 oscillation (see the front and back at least a group of two high-reflection and/or high reflectivity (HR) mirror(s) 302 illustrated in FIG. 3A; these HR mirror(s) 302 make the idler beam 104 resonate between the mirrors).
FIG. 2 illustrates a schematic diagram of a terahertz parametric generator 200 (TPG). The pulsed laser stages in this embodiment can include a neodymium-doped yttrium aluminium garnet Nd:YAG; (Nd:Y3Al5O12) pulsed laser stage and/or a pulsed laser source (hereafter referred to as the “Nd:YAG pulsed laser 202”), which includes a crystal that is used as a lasing medium for solid-state lasers. The frequency of the terahertz beam is tuned by changing an angle phi 103 (φ) between the optical axis 204 of the nonlinear optical crystal 214 and the pump beam 102, which can be accomplished by rotating, mechanically, manually and/or electrically, the optical axis 204 by a rotation stage 212 mounted with the nonlinear optical crystal 214 aligning the pump beam 102.
According to exemplary embodiments, the Nd:YAG pulsed laser 202 disclosed herein can have a wavelength lambda (λ) equal to 1064.7 nanometers (nm).
According to exemplary embodiments, the pulsed energy of the Nd:YAG pulsed laser 202 should be more than 16 millijoules (mJ) associated with the production of a terahertz beam.
According to exemplary embodiments, the pulse width of the Nd:YAG pulsed laser 202 can be between a range of about 17.3 to about 24 nanoseconds (ns); and the repetition rate can be about 20-100 Hertz (Hz) or higher.
Further according to exemplary embodiments, the idler beam 104 can have a wavelength lambda of between about 1065 to about 1075 nm.
According to exemplary embodiments, the pump beam 102 can have a wavelength lambda of about 1064.7 nm.
Additionally, in exemplary embodiments, the length of the nonlinear optical crystal 214 is typically about 65 millimeters (mm).
FIG. 3A illustrates a schematic diagram of a terahertz parametric oscillator 300 (TPO).
Again referring to FIG. 3A, and in regard to the TPO, the idler beam 104 resonates between the HR mirror(s) 302 to increase its amplitude. The pump beam 102 is used once and then dumped into the beam dump 210, both in the TPG and the TPO. The pump beam 102, which is dumped in the beam dump 210, still has substantial laser energy; hence the energy efficiency of the TPG and the TPO techniques is low. In fact the energy conversion efficiency (wherein the energy conversion efficiency is defined as the ratio of the pump beam energy to the terahertz beam energy) of these techniques is below 10−5.
Referring to FIG. 3A, the repetition rate of Nd:YAG pulsed laser 202 is very slow (see the bottom panel in FIG. 3A), making the averaged terahertz output power low.
Additionally, when the power density of the Nd:YAG pulsed laser 202 exceeds 1.2×108 W/cm2, the pump beam 102 can damage the nonlinear optical crystal 214. Therefore, the intensity of the pump beam 102 should be limited to be below a certain value (i.e., a power density of about 1.2×108 W/cm2 for the exemplary embodiments) to prevent damage to the nonlinear optical crystal 214.
Due to this limitation on the pump power and also due to the slow repetition rate of pulses of the pump beam 102, the averaged output power of either the TPG and/or the TPO illustrated in FIG. 2 and FIG. 3A respectively is very low (typically below 100 μW).
Referring to FIG. 3A again, and the terahertz parametric oscillator 300, the pair of high-reflection (HR) mirror(s) 302 (hereafter referred to as the “HR mirror(s) 302”) make the idler beam 104 resonate between the minors. Similar to the terahertz parametric generator, the terahertz frequency is tuned by rotating the nonlinear optical crystal 214 (i.e., changing the angle phi 103 (φ) between the optical axis 204 of the nonlinear optical crystal 214 and the pump beam 102).
New applications of terahertz technology have been theorized and experimentally demonstrated in the laboratory environments, including the detection of hidden weapons, explosives, biochemical agents, and nuclear materials.
Practical implementations of these new applications, have not been possible until the development of the exemplary embodiments, because conventional terahertz sources are limited to low output power and because current terahertz detectors have low sensitivities. Free electron lasers have tunable capabilities and they can produce high power terahertz beams ranging from about 100 watts to about a few kilowatts; but, free electron lasers work in a manner different than the exemplary embodiments and free electron lasers are too bulky and too heavy to be useful in the field. Thus, there are no known portable, tunable, high-output-power, terahertz beam sources comparable to the exemplary embodiments disclosed herein.
Therefore, the need exists for portable, high-power terahertz-beam sources which are tunable and have high efficiency.